Hi Pietro,
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I too had an issue with instable power and swr with my FT-817, which took me quite some time to track down.
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The final unit has some kind of gray shielfing foil on top. With the shielding removed, the power and swr on a dummy load were stable.
After assemling the shielding and the case, the erratic behavior started again.
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In the end an additional insulating piece of a plasic foil between the pa board and the gray shieling foil cured it for my unit.
It may or may not be a solution fo your unit, but it would be a simple try.
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Good luck, 73
Chris
